Hydrogen-occlusion electrode and a method of manufacturing the electrode

ABSTRACT

A hydrogen-occlusion electrode and a method of manufacturing the electrode are described. The hydrogen-occlusion electrode comprises from about 42-84 vol. % of a hydrogen-occlusion alloy, from about 3-13 vol. % of a bonding material, from about 3-15 vol. % of an electroconductive material, and has residual pores in an amount of 10-30 vol. %. The electroconductive material has an average particle size of 1.3 μm or less. The bonding material is preferably polyvinylidene fluoride (PVdf). In manufacturing the foregoing hydrogen-occlusion electrode, a formed electrode plate body including PVdF as a bonding material is heat-treated in a vacuum or inert gas atmosphere at a temperature ranging from about 160° C. to about 200° C. An electrode is thereby obtained wherein the alloy particles are prevented from separating from the electrode resulting in an improvement of mechanical strength and electroconductivity in the electrode. When the hydrogen-occlusion electrode includes a thickener in an amount of not more than about 0.5 wt. % based on the weight of the hydrogen-occlusion alloy, improvements in the charge-discharge characteristics and the internal pressure of the battery are obtained.

This is a division of application Ser. No. 08/074,452 filed Jun. 10,1993, now U.S. Pat. No. 5,360,687.

FIELD OF INVENTION

The invention relates to a hydrogen-occlusion electrode and a method ofmanufacturing such electrode.

BACKGROUND OF INVENTION

A hydrogen-occlusion electrode primarily composed of ahydrogen-occlusion alloy capable of occluding and releasing hydrogen haspreviously been proposed for use as a negative electrode of asealed-type alkaline storage battery in which hydrogen is used as theactive material for the negative electrode. To manufacture this type ofhydrogen-occlusion electrode, a polytetrafluoroethylene (PTFE) powder ora polyethylene (PE) powder, which serves as a bonding material, is mixedwith a hydrogen-occlusion alloy powder which is the main component ofthe electrode. The mixture is heated sufficiently to bond the alloypowder particles together. Alternatively, an unsintered PTFE powder ismixed with a hydrogen-occlusion alloy powder and formed into fibers.During the formation of these fibers, particles of the alloy powder arebonded to one another in order to prevent the alloy powder particlesfrom separating from the formed fibers. In this case, it has been thegeneral practice to add an electroconductive material, such as a nickel(Ni) powder, to the mixture to increase the conductivity of theelectrode. Further, a thickener, such as carboxymethylcellulose (CMC) isadded to the mixture to make the mixture a slurry. The resultant slurrymixture can then be applied to a porous or perforated electroconductivesubstrate, dried and pressed to a predetermined thickness to form anelectrode plate body. This body is thereafter heat-treated in a vacuumor inert gas atmosphere to produce a hydrogen-occlusion electrode.

With the types of hydrogen-occlusion electrodes manufactured by theabove-identified conventional manufacturing methods, however, it hasbeen found that as the electrode is repeatedly subjected tocharge-discharge operations in an alkaline electrolyte while in use as anegative electrode of a storage battery, pulverization of thehydrogen-occlusion alloy powder contained in the electrode takes placeresulting in finer-sized particles of the hydrogen alloy powder whichseparate from the electrode. This results in not only a capacitydecrease in the battery, but also a sharp deterioration in both themechanical strength and electroconductivity of the electrode, therebymaking it difficult to maintain a high capacity rate for a long time.

Further, it has been the general practice with conventionhydrogen-occlusion electrodes containing a thickener, that the thickenerbe present in an amount of about 1 wt. % based on the weight of thehydrogen-occlusion alloy of the electrode. However, this has been foundto cause the surfaces of the alloy particles to become covered with thethickener resulting in lower electrode activity, a comparatively shortlife in terms of charge-discharge cycles, and a higher internal pressurefor the sealed-type storage battery containing the electrode. It isdesired to avoid these disadvantages.

OBJECTS AND BRIEF DESCRIPTION OF THE INVENTION

A primary object of the present invention is to provide ahydrogen-occlusion electrode which avoids the above-describeddisadvantages and has an improved charge-discharge cycle life, capacityretention or preservation rate, and the like.

A hydrogen-occlusion electrode according to the present invention ischaracterized in that it comprises, based on the volume percentage ofthe electrode, a mixture of from about 42-84 volume % of ahydrogen-occlusion alloy, from about 3-13 volume % of a bondingmaterial, from about 3-15 volume % of an electroconductive material, andincludes from about 10-30 volume % of residual pores. The preferredbonding material is polyvinylidene fluoride (PVdf) and theelectroconductive material preferably has an average particle size of1.3 μm or smaller. The electrode components are heat-treated followingmixture to form the electrode.

Another object of the present invention is to provide a method ofmanufacturing a hydrogen-occlusion electrode which avoids thedisadvantages of prior electrodes as above described and improves theproperties of the produced battery, such as charge-discharge cycle life,discharge characteristics, and the like.

The method of manufacturing a hydrogen-occlusion electrode according tothe present invention is characterized in that an electrode plate bodyis formed using polyvinylidene fluoride as a bonding material, and theformed electrode plate body is heat-treated in a vacuum or inert gasatmosphere at a temperature in the range of about 160° C. to about 200°C.

Further, another object of the present invention is to provide ahydrogen-occlusion electrode which avoids the disadvantages of priorelectrodes as above described by manufacturing an electrode according tothe following method: (1) mixing a hydrogen-occlusion alloy powder as amain component, an electroconductive material, a bonding material and athickener together with water to make a slurry mixture, (2) applying theresultant slurry mixture to an electroconductive porous or perforatedsubstrate, and (3) drying and pressing the substrate, wherein thethickener is present in an amount of not more than about 0.5 wt. % basedon the weight of the hydrogen-occlusion alloy. This method serves toimprove the electrode's charge-discharge cycle life characteristics andthe internal pressure of the sealed-type storage battery containing anelectrode so produced. Preferred thickeners suitable for use includecarboxymethyl-cellulose (CMC), methyl cellulose (MC), polyvinyl alcohol(PVA), hydroxy-propylmethyl cellulose (HPMC), polyethylene oxide (PEO)or the like. In use, the thickener which is present in an amount of fromabout 0.01 to about 0.5 wt. % based on the weight of thehydrogen-occlusion alloy is preferably dissolved in water or an organicsolvent and used in the form of a solution.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing the charge-discharge characteristics ofhydrogen-occlusion electrodes manufactured according to the presentinvention and electrodes produced for comparison purposes.

FIG. 2 is a graph showing the improvement effects obtained on thecharge-discharge characteristics of batteries which containhydrogen-occlusion electrodes of the present invention.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

Hydrogen-occlusion electrodes and methods of manufacturing theelectrodes according to the present invention are described belowincluding through specific examples of preferred embodiments.

A hydrogen-occlusion alloy useful for use in manufacturing ahydrogen-occlusion electrode according to the present invention can bean alloy of a misch metal-nickel (MmNi) system or of any other knowncomposition suitable for the desired use in an electrode. Misch metal asused herein refers to a mixed rare earth metal prepared by electrolysisof fused rare earth chlorides. It has been found that in making ahydrogen-occlusion electrode according to the present invention thatwhen the compounding ratio of the alloy in the electrode is less than 42vol. %, a high capacity of the electrode cannot be obtained. Further,when the compounding ratio of the alloy is in excess of 84 vol. %, therelative quantities of the bonding material and the electroconductivematerial have to be decreased resulting in a less secure bonding of thealloy particles and a lowering of the electrode's electroconductivity.Using polyvinylidene fluoride (PVdF) as the bonding material in thehydrogen-occlusion electrode has been found to be extremely advantageousin that it is alkali-resistant and can be heat-treated sufficiently at atemperature in the range of 160° C. to 200° C. as compared with theconventionally used polytetrafluoroethylene (PTFE) which requires a muchhigher heating temperature as well as a special expensive furnace forits heat-treatment since its sintering temperature is as high as about350° C. However, PTFE can be used as the bonding material within thedescribed amounts and improvements obtained over conventional batteries.The most preferred bonding material is PVdF since this allows for use oflower processing temperatures.

Following formation of an electrode plate body using polyvinylidenefluoride and a hydrogen-occlusion alloy powder, the electrode plate bodyis heated at a temperature in the range of about 160° C. to about 200°C. to sinter the particles of the PVdf bonding material. This results inthe bonding together or formation of a network of the particles ofbonding material and the secure bonding of the particles of the powderedalloy one to another by means of the network of the bonding material.This structure has been found to accommodate the expansion andcontraction in volume of the alloy and is very advantageous in use. Whenthe heat treatment temperature is lower than about 160° C., particles ofthe bonding material have not sufficiently bonded to one another and, asa result, fine particles of the powdered alloy are not held securely inposition by the bonding material. When the heating temperature is higherthan about 200° C., undesirable surface inactivation of the alloyparticles is generally caused, possibly due to a flowing of the bondingmaterial around the alloy particles.

Suitable electroconductive materials for use in the hydrogen-occlusionelectrode include any conventionally known electroconductive material asused in sealed-type storage batteries which provide goodelectroconductivity. Carbonyl nickel is preferred. For example, nickelpowders produced by a carbonyl refining process and sold commercially byINCO Specialty Powder Products. As will be discussed further below, whenthe average particles size of the electroconductive material is largerthan 1.3 μm, it is difficult for the material to enter the spacesbetween the alloy particles and sufficiently bond to the surfaces of thealloy powder, thereby resulting in a lower coefficient of utilizationfor the electrode.

The thickener used for making a slurry mixture of the powdered alloy,bonding material and electroconductive material, can be anyconventionally known thickener, such as CMC, MC, PVA, HPMC, PEO and thelike. Further, the thickener is preferably present in solution usingeither water or an organic solvent. This assists in mixing thecomponents present.

Thus, a slurry having a predetermined viscosity can be prepared from amixture which preferably comprises a desired hydrogen-occlusion alloypowder, a bonding powder, an electroconductive powder, a thickenerpowder and a suitable amount of water or an organic solvent. The slurrymixture is applied to both sides of a porous or perforatedelectroconductive substrate so as to fill in the openings in thesubstrate and/or form thereon a paste layer of an appropriate thickness.The pasted substrate is then dried and roll-pressed to produce a formedelectrode plate body having a predetermined thickness. Thereafter, theformed electrode plate body is put into a heating chamber, such as afurnace, and heated in a vacuum atmosphere or in an inert gas atmosphereof nitrogen, argon or the like at a temperature ranging from about 160°C. to about 200° C. for a required length of time. A hydrogen-occlusionelectrode is thereby made having residual pores in an amount of 10 to 30vol. %. Where the residual pores account for less than 10 vol. %, gasabsorption by the electrode is poor and where the residual pores accountfor more than 30 vol. %, the active material is likely to separate fromthe electrode and a high capacity cannot be obtained.

Examples of a preferred embodiment of the present invention is describedbelow.

EXAMPLE 1

A hydrogen-occlusion powder was obtained by mechanically pulverizing ahydrogen-occlusion alloy comprising MmNi₃.5 Co₁.0 Al₀.5, a PVdF powderserving as a bonding material, and a carbonyl nickel powder having anaverage particle size of 1.3 μm serving as an electroconductivematerial, and mixing in the ratios set forth in terms of volumepercentage in Table 1 below. A predetermined quantity of a 1% watersolution of CMC serving as a thickener was added to each of the mixturesprepared above and agitated until the mixture components were uniformlymixed together to provide a slurry of the mixture. Each of the slurrymixtures obtained was applied to both sides of a porous nickel substratesheet and then dried and roll-pressed to form a hydrogen-occlusionelectrode plate body. Each of the formed electrode plate bodies was thenput into a furnace and heated under a vacuum atmosphere at a temperatureof about 170° C. for two hours. Thus were manufactured thehydrogen-occlusion electrodes A through Q, respectively, as shown inTable 1.

Further, for purposes of comparison, a hydrogen-occlusion electrode Rwas manufactured in the same manner as described above except that apolyethylene (PE) powder was added instead of the PVdF powder as thebonding material. Additionally, a hydrogen-occlusion electrode S wasmanufactured using a PTFE powder instead of the PVdF powder as thebonding material and no CMC thickener was added to the mixture. Thismixture was agitated so that the PTFE powder was formed into fibers. Themixture containing the fibrous PTFE was then applied and press-bonded toboth sides of a porous nickel substrate sheet to form an electrode platebody which was not subjected to heat treatment. Further, anotherhydrogen-occlusion electrode T was manufactured in the same manner asdescribed above except that a carbonyl nickel powder having an averageparticle size of 2.8 μm was used as the electroconductive material inplace of the carbonyl nickel powder having an average particle size of1.3 μm.

The residual porosity in terms of volume percentage for each of theelectrodes A through T was as set forth in Table 1.

                  TABLE 1                                                         ______________________________________                                        (Units in Vol. %)                                                                                BOND-    ELECTRO-                                                 HYDROGEN-   ING      CONDUC-   RESID-                                  ELEC-  OCCLUSION   MATE-    TIVE      UAL                                     TRODE  ALLOY       RIAL     MATERIAL  PORES                                   ______________________________________                                        A      69          3        8         20                                      B      67          5        8         20                                      C      64          8        8         20                                      D      61          11       8         20                                      E      59          13       8         20                                      F      69          8        3         20                                      G      67          8        5         20                                      H      61          8        11        20                                      I      57          8        15        20                                      J      74          8        8         10                                      K      69          8        8         15                                      L      71          1        8         20                                      M      57          15       8         20                                      N      71          8        1         20                                      O      54          8        18        20                                      P      79          8        8          5                                      Q      49          8        8         35                                      R      64          8        8         20                                      S      64          8        8         20                                      T      64          8        8         20                                      ______________________________________                                    

Each of the electrode plates A through T was used as a negativeelectrode and stacked together with a nickel electrode plate serving asa positive electrode in laminate fashion with a nylon separator as thinas 0.18 mm interposed therebetween to form a battery element. Thebattery element was rolled up to make a spiral battery element. Thisspiral battery element was placed into a conventional nickel-platedsteel cylindrical container. A conventional alkaline electrolyte aqueoussolution was poured into the container and a cover was attached andhermetically sealed thereto resulting in a sealed-type cylindricalstorage battery. The positive nickel electrode plate described above wasmanufactured by mixing a nickel hydroxide powder with a carbonyl nickelpowder and combining the mixture with a 1.2% water solution of CMC toprovide a slurry mixture. This slurry mixture was applied to a foamnickel substrate to fill up the pores of the substrate. The substratewas then dried and roll-pressed to manufacture the positive electrodeplate. Storage batteries were manufactured as described above usingelectrode plates A through T as the negative electrode and are referredto herein as batteries A through T.

A charge-discharge cycle test, an internal pressure test, and adischarge test using different discharge rates were carried out onbatteries A through T as set forth below. Batteries A through T were allAA-type, 1100 milliampere-hour (mAh) batteries.

CHARGE-DISCHARGE CYCLE TEST

For the charge-discharge cycle test, each battery was charged with 1100milliampere (mA) current for 75 minutes and discharged with 1100 mAcurrent to a final voltage of 1 volt (V). This test was carried out atroom temperature. The test results are shown in FIG. 1.

As clearly seen from FIG. 1, batteries A through K had a very small dropin capacity even over the progression in number of charge-dischargecycles, whereas batteries L through T had a very large drop in capacityover the progression in number of charge-discharge cycles. The cause ofthe capacity drop as shown in FIG. 1, is believed to be due to the factthat the compounding amount of the bonding material is too low as seenin the case of negative electrode L of battery L, and the use ofheat-treated molten PE and non-heat-treated or unsintered fibrous PTFEas the bonding material in electrodes R and S, respectively, instead ofheat-treated PVdF. Even though the amount of the bonding material issufficient in the cases of negative electrode R of battery R andnegative electrode S of battery S, it is believed that the resultingdisadvantage was due to the particles of alloy being broken intofiner-sized pieces through the repetition of charge-discharge cycles andtheir subsequent separation from the electrode. Accordingly,deterioration of both the mechanical strength and conductivity of theelectrode also resulted.

On the other hand, when the compounding amount of the bonding materialis too great, as in the case of electrode M of battery M, it has beenfound that the polarization characteristics of the negative electrodedeteriorate and the coefficient of utilization for the negativeelectrode is lowered. Further, when the amount of the electroconductivematerial is too low, as seen in the case of negative electrode L ofbattery L, the electroconductivity thereof is lowered and, accordingly,the coefficient of utilization is lowered. Conversely, when the amountof the electroconductive material is too great, as in the case ofnegative electrode O of battery O, there has been observed noimprovement proportional to the increased amount with respect of theelectroconductivity and, in fact, a shorter battery life in terms ofcharge-discharge cycles results. The specific reason for this has notyet been determined. Further, when the amount of the residual pores istoo low, as with negative electrode P of battery P, it is believed thatthe gas absorptivity of the electrode is poor thereby preventing themaintenance of a good charge-discharge characteristic for a long periodof time and easy deterioration. Conversely, when the residual porosityis too great, as seen with negative electrode Q of battery Q, it hasbeen found that the alloy particles are susceptible to separating fromthe electrode more easily and the coefficient of utilization for theelectrode lowers sharply. Further, when the average particle size of theelectroconductive material is too large, even when a sufficientcompounding amount is used as seen with negative electrode T of batteryT, the electroconductivity and the coefficient of utilization of theelectrode deteriorates.

In contrast with the deficient negative electrodes described above, thenegative electrodes A through K of batteries A through K, havingcomposition ratios as shown in Table 1 and a heat-treated PVdFelectroconductive material having an average particle size 1.3 μm orless, are provided with a network formed by the heat-treated bondingmaterial which firmly holds the fine-sized particles of the alloy powderin position to prevent them from separating from the electrode and atthe same time reinforcing the mechanical strength of the electrode.Further, the fine-sized particles of the electroconductive materialadhere to the surfaces of the fine-sized alloy particles as if to platethe latter particles thereby assuring good electroconductivity whichresults in a satisfactory coefficient of utilization for the electrode.Additionally, the porosity of the electrode due to the residual pores,as set forth in volume percentages in Table 1, bring about good gasabsorptivity, good infiltration and diffusion of the electrolyte intothe electrode, and good occlusion and release of hydrogen in conjunctiontherewith resulting in the maintenance of the capacity over theprogression in number of charge-discharge cycles at a level essentiallyas high as that present in the initial charge-discharge operation.

INTERNAL PRESSURE TEST

The internal pressure test was carried out so that all the batterieswere charged with 1100 mA current for 4.5 hours and discharged with 220mA current to a final voltage of 1 V. The charge-discharge operation wascarried out at 20° C. The test results are shown in Table 2. Theinternal pressure is set forth in terms of kilogram-force per centimetersquared (kgf/cm²).

                  TABLE 2                                                         ______________________________________                                                INTERNAL                INTERNAL                                              PRESSURE                PRESSURE                                      BATTERY (kgf/cm.sup.2)                                                                              BATTERY   (kgf/cm.sup.2)                                ______________________________________                                        A       5.2           L         15.1                                          B       5.1           M         25.3                                          C       4.8           N         11.2                                          D       6.5           O         13.6                                          E       8.0           P         21.0                                          F       5.5           Q         30.2                                          G       5.0           R         16.7                                          H       6.9           S         14.4                                          I       7.0           T         13.2                                          J       7.5                                                                   K       5.8                                                                   ______________________________________                                    

As clearly seen from Table 2, batteries A through K all had low internalpressure whereas batteries L through T had remarkably high internalpressure when overcharged with a high current. This difference ininternal pressure is believed to be due to the use of a differentcomponent ratio for each electrode as shown in Table 1, the use of adifferent bonding material and different electroconductive material, andwhether the electrode material was heat-treated or not. All of these arebelieved to be factors causing differences in the battery propertiesobtained, such as hydrogen occlusion-release performance,charge-discharge performance, electroconductivity attributable to theelectroconductive material used, bonding characteristics, and gasabsorptivity.

DISCHARGE TEST USING DIFFERENT DISCHARGE RATES

The discharge test was carried out so that all the batteries werecharged with 220 mA for 7.5 hours and discharged to a final voltage of 1V using three different discharge rates, namely, 220 mA [0.2Coulomb(C)], 1650 mA (1.5 C) and 3300 mA (3.0 C). This test was carriedout at 20° C. The test results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        (Units in mAh)                                                                BATTERY    0.2 C         1.5 C  3.0 C                                         ______________________________________                                        A          1198          1078   982                                           B          1190          1055   975                                           C          1189          1080   990                                           D          1150          1022   966                                           E          1144          1011   956                                           F          1177          1063   973                                           G          1170          1062   977                                           H          1172          1067   970                                           I          1169          1065   971                                           J          1174          1058   968                                           K          1178          1064   963                                           L          1189          1055   961                                           M          1155           924   751                                           N          1160           986   812                                           O          1180          1060   965                                           P          1183          1063   973                                           Q          1130           920   740                                           R          1166          1014   898                                           S          1173          1009   889                                           T          1159           989   869                                           ______________________________________                                    

As clearly seen from Table 3, batteries A through K had a very smalldrop in their capacity even with a very high current as compared withbatteries L through T. Batteries A through K have good dischargecharacteristics.

Thus, batteries A through K had good results with respect to each of thecharge-discharge cycle test, the internal pressure test and thedischarge test using different discharge rates.

Further, it has been found that a hydrogen-occlusion electrode for astorage battery can be obtained having improved charge-discharge cyclecharacteristics and internal pressure characteristics when containing athickener in an amount of not more than about 0.5 wt. % based on theweight of the hydrogen-occlusion alloy contained in the electrode.

A hydrogen-occlusion alloy contained in a hydrogen-occlusion electrodeserves to occlude and release hydrogen in an aqueous solution ofalkaline electrolyte during charge and discharge of the battery. Suchocclusion and release of hydrogen take place right on the surfaces ofthe alloy which are exposed to the electrolyte. According to the presentinvention, when the thickener content in the electrode is limited to notmore than about 0.5 wt. %, the exposed surfaces of the alloy areincreased as compared with a conventional electrode in which about 1 wt.% of a thickener based on the weight of the hydrogen-occlusion alloy isused. Accordingly, when a thickener in an amount of the presentinvention is used, a longer charge-discharge cycle life, a highercapacity retention or preservation rate, and a lower internal pressureof a sealed-type alkaline storage battery is achieved.

Since the content of the thickener contained in the hydrogen-occlusionelectrode manufactured according to the present invention is very low,the surfaces of the particles of hydrogen-occlusion alloy are hardlycovered with the thickener, so that the area of active surfaces exposedto the electrolyte are increased. Due to this increase in the activesurface area performing the occlusion and release of hydrogen, there isbrought about favorable effects such as a longer battery life in termsof charge-discharge cycles, a higher capacity retention rate and a lowerinternal pressure in a sealed-type alkaline storage battery containingan electrode with a thickener in an amount of not more than about 0.5wt. %. While it has been found that the effects as noted above can beobtained when the compounding ratio of a thickener is up toapproximately 0.5 wt. % based on the weight of the hydrogen-occlusionalloy in the electrode, it has also been found that when the thickenercontent is less than 0.01 wt. %, that it is difficult to handle themixture adequately. Therefore, it is preferable to use the thickener inan amount of at least about 0.01 wt. % based on the weight of the alloy.

Specific examples are set forth below showing the advantages obtained inusing a thickener according to the present invention.

EXAMPLE 2

A hydrogen-occlusion alloy composed of MmNi₃.5 Co₁.0 Al₀.5 waspulverized into fine particles to produce a hydrogen-occlusion alloypowder. A nickel powder, as commercially sold under the tradename INCOType-210, in an amount of 15 wt. % based on the weight of the alloypowder was added to the alloy powder as the electroconductive material.Additionally, a PVdF powder in an amount of 3 wt. % based on the weightof the alloy powder was added to the mixture. As the thickener, a CMCpowder in an amount of 0.10 wt. % based on the weight of the alloypowder was dissolved in water to prepare an aqueous solution of thethickener which was then added to the mixture. The components were mixeduniformly and kneaded together to obtain a slurry mixture. This slurrymixture was applied to both sides of a perforated nickel substratesheet, dried and roll-pressed to obtain a formed electrode plate body.This electrode plate body was put into a furnace where it was heated at170° C. for 2 hours in a vacuum atmosphere to manufacture ahydrogen-occlusion electrode plate. This electrode plate is designatedherein as "Electrode A".

Additionally, except that the CMC thickener was used in an amount of 0.5wt. % based on the weight of the hydrogen-occlusion alloy powder, thesame conditions and process as described above with respect to ElectrodeA were carried out to manufacture another hydrogen-occlusion electrodeplate. This electrode is designated herein as "Electrode B".

Further, except that the CMC thickener was used in an amount of 0.80 wt.% based on the weight of the hydrogen-occlusion alloy powder, the sameconditions and process as described above with respect to Electrode Awere carried out to manufacture another hydrogen-occlusion electrodeplate which is designated herein as "Comparison Electrode C".

Each of Electrode A and Electrode B of the present invention andComparison Electrode C were then used as a negative electrode. Eachnegative electrode was stacked in a laminate fashion with a positiveelectrode having a 18 mm thick nylon separator interposed between thepositive and negative electrodes. This laminate was then wound into aspiral form to obtain a spiral battery element. Each of the respectivebattery elements was inserted into a cylindrical nickel-plated steelcontainer. A predetermined quantity of a conventional alkalineelectrolyte was poured into the battery container and a cover was thenattached and hermetically sealed to the container to provide acylindrical sealed-type storage battery. The positive electrode plateutilized was manufactured by (1) mixing a nickel hydroxide powder with acommercially available INCO Type-255 Ni powder and a water solution ofCMC to make a slurry mixture, (2) applying the mixture to a foam nickelsubstrate plate so as to fill up the pores of the substrate, and (3)drying and roll-pressing to a predetermined thickness to produce apositive electrode plate.

Using battery A provided with Electrode A, battery B provided withElectrode B, and battery C provided with Comparison Electrode C, acharge-discharge cycle test and an internal pressure test were carriedout as described below. The batteries tested were AA-size batterieshaving a rated capacity of 1000 mAh.

CHARGE-DISCHARGE CYCLE TEST

For the charge-discharge cycle test, each battery was charged with 1ampere (A) current for 75 minutes and discharged with 1 A current to afinal voltage of 1 V. This test was carried out at room temperature. Thetest results are shown in FIG. 2.

As clearly seen from FIG. 2, battery C had a large decrease in itscapacity over the progression of the number of charge-discharge cycles,whereas battery A and battery B showed little decrease in capacity andmaintained a 90% or higher capacity retention rate even at the 200thcycle of the charge-discharge operation. The reason for this differenceis believed to be due to the fact that Electrode C contained a greateramount of the CMC thickener in terms of weight percentage. NegativeElectrode C would have had almost all the surfaces of the particles ofpowdered alloy therein covered with CMC thereby leaving a much smallerarea of exposed surface to occlude and release hydrogen. Accordingly,battery C failed to effectively cope with the high-rate current of 1 Aused in the charge-discharge operation. Battery A and battery B wouldhave had much smaller portions of the surfaces of the particles ofpowdered alloy covered with the CMC thickener because they eachcontained lower amounts of CMC thereby leaving a greater surface areaexposed to the electrolyte. Accordingly, a high surface activity capableof occluding and releasing hydrogen efficiently, even at the times ofcharge and discharge operation with a high-rate current of 1 A, was ableto be maintained.

INTERNAL PRESSURE TEST

For the internal pressure test, the batteries were charged with 1 Acurrent for 4.5 hours and discharged with 0.2 A current to a finalvoltage of 1 V. This test was carried out at a temperature of 20° C. Thetest results are shown in the Table 4.

                  TABLE 4                                                         ______________________________________                                                    Internal pressure                                                 Battery     (kgf/cm.sup.2)                                                    ______________________________________                                        A           6                                                                 B           6                                                                 C           12                                                                ______________________________________                                    

As clearly seen from Table 4, battery A and battery B had a much lowerinternal pressure as compared with battery C. It is therefore believedthat, in the case of Electrode A and Electrode B, more surface area ofthe hydrogen-occlusion alloy particles was not covered by CMC andtherefore a greater surface area of the alloy remained exposed allowingfor a more efficient absorption of the oxygen generated from thepositive electrodes while at the same time having a higher chargeefficiency. All of these actions combine to prevent the negativeelectrode from generating pressure within the battery and to maintain alower internal pressure.

As compared with the above, it is believed that in battery C thehydrogen-occlusion alloy particles in the negative electrode had almostall of their surfaces covered with CMC leaving only a very small portionexposed to the electrolyte. Accordingly, oxygen generated from thepositive electrode could not be absorbed efficiently and a decrease incharge efficiency of the negative electrode was caused. These effectscombined resulted in a remarkably high rise in internal pressure.

Therefore, as shown, when the content of the thickener in ahydrogen-occlusion electrode is not more than about 0.5 wt. %, based onthe weight of the hydrogen-occlusion alloy contained in the electrode, asealed-type storage battery using this electrode will have improvedcharge-discharge characteristics and a lower internal pressure.

As will be apparent to one skilled in the art, various modifications canbe made within the scope of the aforesaid description. Suchmodifications being within the ability of one skilled in the art form apart of the present invention and are embraced by the appended claim.

It is claimed:
 1. A method of manufacturing a hydrogen-occlusionelectrode comprising (1) forming an electrode plate body using anelectrode comprising, based on volume percentage of said electrode, amixture of from about 42-84 volume % of a hydrogen-occlusion alloy, fromabout 3-13 volume % of a polyvinylidene fluoride bonding material, andfrom about 3-15 volume % of an electroconductive material having anaverage particle size of about 1.3 μm or smaller, wherein said electrodehas from about 10-30 volume % of residual pores; and (2) heat treatingsaid electrode plate body in a vacuum or inert gas atmosphere at atemperature in the range of from about 160° C. to 200° C.